[0001] The present invention relates to an encoder and a signal adjustment method for the
encoder. More particularly, the present invention relates to an encoder that enables
easy signal adjustment and is suitable for use in a separate type absolute (ABS) linear
encoder in which a scale and a detection head are provided separately or rotary encoder,
and also relates to a signal adjustment method for that encoder.
[0002] An encoder for detecting a position from two-phase analog signals formed by a sine
wave and a cosine wave generally includes an interpolation (division) circuit in order
to detect a change of the position (the moving amount) that is smaller than a period
of the signal, as described in Japanese Patent Laid-Open Publication No. Sho 54-19773.
The encoder equally divides the signal period at a predetermined pitch so as to obtain
resolution smaller than the signal period.
[0003] The interpolation circuit calculates a phase angle θ from the following expression,
assuming that the sine wave (phase B) signal and the cosine wave (phase A) signal
output from the encoder have a central voltage, amplitude, and a phase difference
that are predetermined, as shown in Fig. 1.
[0004] However, in the case where the central voltage, amplitude, or phase difference of
at least one of the sine and cosine waves is not coincident with the predetermined
value, the detected phase angle (the change amount of the position equal to or smaller
than the signal period) does not have a constant pitch and an error (interpolation
error) coincident with the signal period occurs, as shown in Figs. 2 to 5. Fig. 2
shows an example of the interpolation error in the case where the phase A signal has
an offset of 0.2. Fig. 3 shows an example of the interpolation error in the case where
the amplitudes of the phase A signal and the phase B signal are 1.0 and 1.2, respectively.
That is, Fig. 3 shows a case where there is an amplitude difference between the phase
A signal and the phase B signal. Fig. 4 shows an example of the interpolation error
in the case where the phase B signal has a phase error of 10°. Fig. 5 shows an example
of the interpolation error in the case where the phase A signal has the amplitude
of 1.0 and the offset of 0.05 and the phase B signal has the amplitude of 0.95, the
offset of -0.05 and the phase error of -5°. That is, Fig. 5 shows the case where the
offset, the amplitude difference, and the phase error occur at the same time.
[0005] Thus, in case of using a conventional encoder, it is necessary to adjust a signal
from the encoder to have a central voltage, amplitude, and a phase difference that
are predetermined, while observing the signal (e.g., a Lissajous waveform of the phase
A signal and the phase B signal) on an oscilloscope.
[0006] The adjustment while observing a display screen of the oscilloscope requires skills.
In order to overcome this problem, an encoder is proposed which includes a comparator
for determining whether or not a signal having a predetermined level is obtained and
outputting a determination result indicating that the determined signal is not an
optimum signal inside (or outside) the encoder, as proposed in Japanese Patent No.
3202316. In this case, it is possible to confirm whether or not a predetermined signal
is obtained, without using the oscilloscope.
[0007] However, the encoder described in Japanese Patent No. 3202316 does not have a self-adjustment
function. Therefore, in the case where a range of a signal level in which the comparator
determines the signal as the optimum signal is made smaller, mechanical adjustment
during attachment of the encoder becomes more difficult. On the other hand, in the
case where the above range is made larger, a tolerance of a signal error also becomes
larger, resulting in increase in the interpolation error.
[0008] On the other hand, an ABS encoder that can detect an absolute position is widely
used in machine tool or industrial machinery that includes an encoder for the reasons
that (1) the ABS encoder does not require zero return at starting and (2) the ABS
encoder does not require a magnetic-pole detector of a linear motor when the encoder
is used for feed-back of the linear motor, and other reasons. The ABS encoder includes
a multi-track encoder, and synthesizes signals obtained from multiple tracks so as
to obtain the absolute position.
[0009] Synthesis of the signals from the multiple tracks uses a CPU. An encoder including
the CPU therein has been commonly used.
[0010] The present invention aims to overcome the aforementioned problems of the conventional
techniques. It is an object of the present invention to provide an encoder including
a CPU therein, that enables easy signal adjustment so as to obtain an optimum signal
to be performed without making a Lissajous waveform close to a perfect circle while
observing a display screen of an oscilloscope, or providing a comparator for outputting
a comparison result indicating that a signal of the encoder falls within a predetermined
range or the like in the encoder.
[0011] In order to achieve the object described above, according to an aspect of the present
invention, an encoder comprises: a detector; means for performing A/D conversion for
an analog signal having at least two phases, the signal being output from the detector;
means for correcting an error of the analog signal; an interpolation circuit for performing
interpolation from a result of A/D conversion of the analog signal that is corrected;
means for storing correction data; and central processing means having communication
means.
[0012] The encoder may send the result of A/D conversion of the analog signal to an external
device by the communication means; the external device may detect the error of the
signal from a predetermined value and send the error to the encoder by the communication
means; and the encoder may perform interpolation for which error correction is performed
by using the received correction data.
[0013] In order to achieve the object described above, according to another aspect of the
present invention, an encoder comprises: a detector; means for performing A/D conversion
for an analog signal having at least two phases, the analog signal being output from
the detector; means for detecting an error of the analog signal; means for correcting
the error of the analog signal; an interpolation circuit for performing interpolation
from a result of A/D conversion of the analog signal that is corrected; means for
storing correction data; and central processing means having communication means.
[0014] The encoder may send the result of A/D conversion of the analog signal after correction
to an external device by the communication means, and the external device may display
and determine the corrected signal.
[0015] According to the present invention, it is possible to perform signal adjustment with
high precision without using an oscilloscope. Moreover, a status of signal adjustment
can be confirmed in a non-stepped manner, not by a stepped indication using LEDs as
described in Japanese Patent No. 3202316. Furthermore, it is possible to confirm fine
data, instead of a result of rough adjustment. Therefore, optimum adjustment and confirmation
are possible.
[0016] Especially, when the external device such as a personal computer is made to have
a storage function, adjustment data of each encoder can be collectively managed.
[0017] These and other novel features and advantages of the present invention will become
apparent from the following detailed description of preferred embodiments.
[0018] The preferred embodiments will be described with reference to the drawings, wherein
like elements have been denoted throughout the figures with like reference numerals,
and wherein;
Fig. 1 is a diagram for explaining the principle of interpolation in an encoder;
Fig. 2 is a diagram showing an effect of an offset in order to explain a problem of
a conventional technique;
Fig. 3 is a diagram showing an effect of an amplitude difference in order to explain
the problem of the conventional technique;
Fig. 4 is a diagram showing an effect of a phase error in order to explain the problem
of the conventional technique;
Fig. 5 is a diagram showing a composite effect of the offset, the amplitude difference,
and the phase error in order to explain the problem of the conventional technique;
Fig. 6 is a perspective view showing connection during signal adjustment in a first
embodiment of the present invention;
Fig. 7 is a front view showing final connection in the first embodiment;
Fig. 8 is a block diagram of an internal structure according to the first embodiment;
Fig. 9 is a flowchart of a procedure according to the first embodiment;
Fig. 10 illustrates an exemplary display screen during adjustment in the first embodiment;
Fig. 11 illustrates an exemplary display screen when a communication error occurs
in the first embodiment; and
Fig. 12 is a block diagram of an internal structure according to a second embodiment
of the present invention.
[0019] Preferred embodiments of the present invention will be now described in detail with
reference to the accompanying drawings.
[0020] The first embodiment of the present invention mainly includes: a scale base 10 attached
to one portion of a machine (not shown), that is a scale-integrated type, for example;
a detection head 20 that is fixed to another portion of the machine away from the
scale base 10 by a predetermined distance and includes an LED 21 for indicating power-on
and an alarm; a head cable 22 and an output connector 24 for connecting the detection
head 20 to a feed-back cable 26 and a servo amplifier 28 of the machine after completion
of adjustment, as shown in Fig. 7; and a personal computer (PC) 50 that is connected
to the output connector 24 during adjustment, instead of the servo amplifier 28, as
shown in Fig. 6.
[0021] Fig. 8 shows details of the detection head 20. As shown in Fig. 8, the detection
head 20 includes a 3-track photoelectric type detector 30 having a light-emitting
device and a light-receiving device, for example; an A/D converter 32 for performing
A/D conversion for a sine wave signal and a cosine wave signal that form a two-phase
analog signal output from the detector 30; an error correction circuit 34 for correcting
an error of the aforementioned sine and cosine wave signals (a central voltage difference,
an amplitude difference, and a phase difference); an interpolation circuit 36 for
performing interpolation from the result of A/D conversion of signals corrected by
the error correction circuit 34; communication means 38 for outputting an output of
the interpolation circuit 36 to a control device (servo amplifier 28); a memory 40
for storing correction data; and a central processing unit (CPU) 42 having communication
means 44 for communicating with the personal computer 50.
[0022] In the present embodiment, the result of A/D conversion of the sine wave and the
cosine wave by the A/D converter 32 (i.e., raw data) is sent to the personal computer
50 that is an external device by the communication means 44. The personal computer
50 then detects an error of the sine wave signal and the cosine wave signal from a
predetermined value.
[0023] The error is then sent from the personal computer 50 to the detection head 20 by
the communication means 44. The detection head 20 performs interpolation in which
error correction is performed, by using correction data that is received and stored
in the memory 40. Thus, it is possible to perform optimum signal adjustment and confirmation.
Moreover, the adjusted signal can be confirmed by displaying waveforms obtained by
performing error correction for data of the sine wave and cosine wave that are sent
from the detection head 20, on the personal computer 50.
[0024] A specific procedure is shown in Fig. 9.
[0025] First, the amplitude of a signal is checked in Step 100. When a result of the check
in Step 100 is No, i.e., it is determined that the signal amplitude is insufficient,
the procedure goes to Step 102 in which the position of the detection head 20 is mechanically
adjusted. The adjustment of the signal amplitude may be achieved by additionally providing
an amplifier 46 having a variable gain in the detection head 20 (as shown by a dashed
line in Fig. 8) and adjusting the amplitude of a signal input to the detector 30 by
using an output of the CPU 42. In this case, a range in which the amplitude can be
adjusted can be made larger.
[0026] When the result of the determination in Step 100 is Yes, the procedure goes to Step
104 in which signal adjustment is performed. More specifically, while the detection
head 20 is scanned over the entire length of the scale 10 in accordance with an instruction
displayed on a display screen, for example, it is confirmed whether or not the LED
21 indicates an abnormal state. Based on a lighting state of the LED 21, necessary
adjustment is performed.
[0027] When lighting of the LED 21 does not occur over the entire length of the scale 10
as a result of the adjustment, the signal amplitude is checked again in Step 106.
When there is no problem, the signal adjustment is ended and the detection head 20
is fixed to the other portion of the machine at that position.
[0028] Fig. 10 shows an exemplary display screen of the personal computer 50 during adjustment.
This display screen corresponds to a case where there are three tracks on the scale.
When the detection head 20 is scanned with respect to the scale 10, Lissajous signals
of the respective tracks are displayed.
[0029] In this manner, it is possible to adjust and fix the detection head 20 at an appropriate
position with respect to the scale 10. In the case where no communication is performed
because of a trouble in a power source or a connecting cable, a message indicating
a communication error is displayed, as shown in Fig. 11. Thus, it is possible to urge
an inspecting person to check the power source or the connecting cable.
[0030] In the present embodiment, detection of the error of the sine and cosine wave signals
is performed in the personal computer 50 externally provided. Thus, the structure
of the detection head 20 is relatively simple.
[0031] Next, the second embodiment of the present invention is described. In this embodiment,
the error detection is not performed in the external device, but is performed in the
detection head 20.
[0032] In this embodiment, error detection is performed in the CPU 42 in an arrangement
similar to that in the first embodiment, as shown in Fig. 12.
[0033] Except for the above, the present embodiment is the same as the first embodiment.
Therefore, detailed description of the present embodiment is omitted.
[0034] In the present embodiment, not raw data but an error detected by the CPU 42 is sent
from the communication means 44 to the personal computer 50. Therefore, it is possible
to easily determine whether or not optimum adjustment is performed by displaying and
determining the corrected signal on the personal computer 50.
[0035] Although the present invention is applied to the separate type linear scale in which
the scale and the detection head are provided separately in the aforementioned embodiments,
an application of the present invention is not limited thereto. The present invention
can be used for confirming an adjustment result in a separate type rotary encoder
or an integrated type encoder.
[0036] Moreover, the number of the phases of the analog signal is not limited to two. For
example, the present invention can be applied to an encoder that outputs a three-phase
analog signal with a phase difference of 120°. In this case, the three-phase analog
signal is input to a known three-phase to two-phase conversion circuit so as to obtain
a two-phase analog signal. Then, the thus obtained two-phase analog signal is processed
in a similar manner to that described in the above embodiments.
1. An encoder comprising:
a detector;
means for performing A/D conversion for an analog signal having at least two phases,
the signal being output from the detector;
means for correcting an error of the analog signal;
an interpolation circuit for performing interpolation from a result of A/D conversion
of the analog signal that is corrected;
means for storing correction data; and
central processing means having communication means.
2. The encoder according to claim 1, further comprising means for displaying a status
of adjustment.
3. An encoder according to claim 1 or claim 2, further comprising means for detecting
an error of the analog signal.
4. A method for adjusting a signal of an encoder, the encoder comprising:
a detector;
means for performing A/D conversion for an analog signal having at least two phases,
the signal being output from the detector;
means for correcting an error of the analog signal;
an interpolation circuit for performing interpolation from a result of A/D conversion
of the analog signal that is corrected;
means for storing correction data; and
central processing means having communication means, the method comprising the steps
of:
sending the result of A/D conversion of the analog signal to an external device by
the communication means;
allowing the external device to detect the error of the signal from a predetermined
value and to send the error to the encoder by the communication means; and
allowing the encoder to perform interpolation for which error correction is performed
by using the received correction data.
5. The method for adjusting a signal of an encoder according to claim 4, wherein the
analog signal is a two-phase signal converted from a three-phase signal.
6. The method for adjusting a signal of an encoder according to claim 4 or claim 5, wherein
the external device is a personal computer.
7. The method for adjusting a signal of an encoder according to any of claims 4 to 6,
wherein the signal transmitted from the encoder and a waveform for which the error
is corrected are displayed on the external device.
8. The method for adjusting a signal of an encoder according to any of claims 4 to 7,
wherein the signal adjustment is achieved by mechanically adjusting a position of
the detector or adjusting a gain of an amplifier provided in the detector when an
amplitude of the signal is insufficient; and
then adjusting the signal while scanning the detector.
9. The method for adjusting a signal of an encoder according to any of claims 4 to 8,
wherein a message indicating a communication error is displayed when no communication
is performed.
10. A method according to any of claims 4 to 9, wherein the encoder further comprises
means for detecting an error of the analog signal.